Many of currently available bicycles not only have a multiple freewheel to constitute a rear gear mounted on a rear wheel hub, but also incorporate a multiple chainwheel to constitute a front gear (front gear being usually called "chainwheel") mounted on a pedal crank, thereby increasing the number of selectable speeds. When, for example, a freewheel having five sprockets is combined with a chainwheel having three sprockets, it is possible to select fifteen speeds.
In a multiple chainwheel, a diametrically largest sprocket is located at the right as viewed in the bicycle running direction, and progessively smaller sprockets are arranged toward the left. A maximum speed is obtainable when the chain is in engagement with the largest sprocket.
With such a chainwheel, a speed change is performed by causing a front deraileur to laterally press a portion of the chain entering to the chainwheel in rotation, with the result that the chain is shifted from a smaller sprocket to a larger sprocket or vice versa. Obviously, the speed change performance of the chainwheel is determined by the smoothness and promptness in disengaging the chain from a presently engaging sprocket for engagement with a target sprocket.
However, a problem called "chain locking phenomenon " arises in shifting the chain from a larger sprocket to a smaller sprocket. This phenomenon is now described with reference to FIGS. 9 through 13.
FIGS. 9, 10, 11 and 13 are views, as seen from the left side of the bicycle, showing the successive stages of shifting a chain C from a larger sprocket 1 to smaller sprocket 2 of a chainwheel CW to illustrate the problem. FIG. 12 is a view as seen in the direction of an arrow XII in FIG. 11.
In FIGS. 9, 10, 11 and 13, the larger sprocket 1 is located farther from the viewer. The chain C comprises pairs of inner link plated Pa1, Pa2, and pairs of outer link plates Pb1, Pb2 which are alternate with the pairs of inner link plates and connected thereto by means of roller pins R. The chain C is engageable with each sprocket in a manner such that each tooth of the sprocket is inserted between a corresponding pair of inner link plates Pa1, Pa2 or outer link plates Pb1, Pb2 with the roller pins R received between the sprocket teeth.
In shifting the chain C in engagement with the larger sprocket 1 to the smaller sprocket 2, a portion of the chain C moving into engagement with the larger sprocket 1 is pressed toward the smaller sprocket 2 by an unillustrated front deraileur, thereby causing the chain C to disengage from the larger sprocket 1. The chain C is subjected to a tension applied by a tension spring incorporated in an unillustrated rear deraileur. Therefore, the chain C having disengaged from a tooth ta of the larger sprocket 1 starts shifting to the smaller sprocket 2 in a manner such that the chain C extends tangentially to the smaller sprocket from the roller pin RO (disengagement starting roller pin) immediately following the pair of link plates which have disengaged from the larger sprocket tooth ta, as shown in FIG. 10.
Now, a tangential line 1 for the smaller sprocket 2 is drawn from a teeth interval center 01 of the larger sprocket 1 located at the disengagement starting roller pin RO to a teeth interval center 02 of the smaller sprocket 2. When the length of the tangential line 1 is larger than an integral multiple (n) of the chain pitch (p) by a fragmentary amount x (i.e., the distance between the teeth interval centers 01 and 02 being np +x), the chain locking phenomenon occurs upon application, to the chainwheel CW, of a large rotational force.
Normally, at the time of chain shifting, the chainwheel rotates for a while with laterally displaced roller pins R (following the disengagement starting roller pin RO) of the chain partially engaging teeth tips of the smaller sprocket 2. The chain can come into full engagement with the smaller sprocket 2 only when the laterally displaced roller pins R correspond in position to the teeth intervals of the smaller sprocket. However, if a large rotational force is applied to the chainwheel CW held in the condition of FIG. 10, the chain C is subjected to a large tension. As a result, the laterally displaced roller pins R are forcibly brought into full engagement with the teeth intervals of the smaller sprocket 2, and the disengagement starting roller pin RO corresponding to the above-mentioned teeth interval center 01 of the larger sprocket 1 is forcibly displaced radially inward along a side surface of the larger sprocket 1, as shown in FIG. 11.
As shown in FIG. 12, the inner link plate Pa1 having cleared a tooth ta of the larger sprocket together with the outer link plate Pb1 immediately preceding that particular inner link plate extends obliquely across the interval between the tooth ta and the immediately preceding tooth tb while these link plates are forcibly moved radially inward. The sprocket teeth are rendered thicker toward the teeth roots. Thus, the inner link plate Pa1 and the outer link plate Pb1, when moved radially inward as above, firmly engage with the corresponding teeth ta, tb, so that the chain C is locked to the larger sprocket 1. Further, the inner link plate Pa1 and the outer link plate Pb1 are laterally pressed by the teeth ta, tb, so that the chain is subjected to a bending force in the direction of arrows A in FIG. 12. By the influences of such a bending force, left-hand link plates of the chain C are laterally pressed against the teeth tc, td, te preceding the above-mentioned tooth tb on the side closer to the smaller sprocket, consequently assisting the chain C to be locked to the larger sprocket 1.
Once the chain locking phenomenon occurs, it is no longer possible for the chain C to disengage from the larger sprocket 1. Therefore, the chainwheel CW rotates with the chain C locked to the larger sprocket 1, as shown in FIG. 13. When the chainwheel CW continues to rotate further in this condition, the chain C locked to the larger sprocket 1 comes into damaging impingement with the front deraileur. Further, in case the larger sprocket 1 is made of a light metal, the teeth thereof may be deformed.
The chain locking phenomenon is more likely to occur when each sprocket tooth is made to have a larger width, as shown in FIG. 8. This is because each teeth interval becomes smaller as the tooth width is larger. The reduced teeth interval results in that the link plates of the chain C engage more firmly with the relevant teeth ta, tb, as shown in FIG. 12. Further, the bending force applied to the chain becomes also larger.
Conventionally, therefore, attempts have been often made to slenerize each sprocket tooth by rendering the tooth width to decrease sharply toward the tooth tip, as shown in FIG. 7. Compared with the sprocket of FIG. 8 having larger width teeth, the sprocket having slenderized teeth facilitates chain disengagement and readily allows laterally oblique chain extension, thereby reducing the chance of the chain locking phenomenon.
The conventional countermeasure seems to improve the performance in shifting the chain from the larger sprocket to the smaller sprocket. In reality, however, the conventional measure excessively facilitates chain disengagement. Therefore, a new problem arises that the chain may prematurely disengage from the larger sprocket before completely shifting to the smaller sprocket, whereby the chain falls in between the larger and smaller sprockets. Such premature chain disengagement may be prevented by increasing the width of the sprocket teeth, as shown in FIG. 8. However, this measure in turn leads to increased occurrence of the chain locking phenomenon, as already described.